pubs.acs.org/Langmuir © 2010 American Chemical Society
One Pot Hemimicellar Synthesis of Amphiphilic Janus Gold Nanoclusters for Novel Electronic Attributes P. Biji,†,‡ Nirod K. Sarangi,† and Archita Patnaik*,† †
Department of Chemistry, Indian Institute of Technology, Madras Chennai-600 036, India, and ‡ PSG Institute of Advanced Studies, Coimbatore-641 004, India Received June 10, 2010. Revised Manuscript Received July 17, 2010
A one-pot hemimicellar synthesis of oriented, amphiphilic, and fluorescent Janus gold clusters, establishing the Janus character in terms of ligand asymmetry and distribution, has been demonstrated. The method was based on the efficient Langmuir strategy, where the in situ two-dimensional (2D) reduction of Au3þ in the sprayed micellar electrostatic complex, TOAþ-AuCl4-, was accomplished by subphase tryptophan that acted as the hydrophilic protecting ligand on one hemisphere of the spherical gold cluster. In contrast to the reported micelle-assisted Janus cluster formation, here the cluster growth occurred inside the surface pressure driven hemimicelles, which rapidly formed 2D cluster arrays without any interfacial reorientation. The Janus structure was validated using angle dependent polarized Fourier Transform Infrared Reflection-Absorption Spectroscopy (FT-IRRAS), where orientation dependent vibrational changes in the adsorbed ligand functionalities were detected. Electrochemical impedance measurements of the transferred Janus layers onto hydrophobized ITO revealed the heterogeneous electron transfer rate constant kET to show a clear orientational odd-even parity effect with the odd layers showing much higher rates. Isobaric area relaxation investigations further evidenced toward a hemispherical instantaneous nucleation with edge growth mechanism of the nanoclusters formed at the tryptophan subphase. Surface pressure as a thermodynamic variable effectively controlled the interparticle separation; intercluster electron coupling exhibited insulator-metal transition in the Janus cluster monolayers through scanning electrochemical microscopy investigations.
Introduction Functional assemblies with nanoparticles as building blocks in two- and three-dimensional (2D, 3D) mesoscale geometry and superlattices have commended nanotechnological applications based on their collective properties.1-3 Investigation of the interfacial properties at the air-water interface gives a better insight into the molecular interactions and orientation leading to exploration of varying 2D phase transitions.4,5 LangmuirBlodgett films not only offer the possibility of built-in architectural control at the monolayer level, but also provide useful media for the controlled construction of nanosized particles under ambient conditions.6,7 The method has been used extensively in the preparation of monolayers for molecular electronics and, more recently, to create nanocrystal monolayers with tunable properties at different physical states of reaction media.8 The synthesis of nanoparticles under the ordering influence of organic templates of Langmuir monolayers has received considerable attention.9,10 *To whom correspondence should be addressed. E-mail: archita59@ yahoo.com. Telephone: 0091-44-2257-4217. Fax: 0091-44-2257-4202. (1) Collier, C. P.; Saykally, R. J.; Shiang, J. J.; Henrichs, S. E.; Heath, J. R. Science 1997, 277, 1978–1981. (2) Shipway, A. N.; Katz, E.; Williner, I. ChemPhysChem. 2004, 1, 18–52. (3) Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104, 293–346. (4) Gaines, G. L. Insoluble Monolayers at Liquid-Gas Interfaces; John Wiley & Sons, Inc.: New York, 1966. (5) Kaganer, V. M.; Mohwald, H.; Dutta, P. Rev. Mod. Phys. 1999, 71, 779–819. (6) Yang, J. H.; Chen, Y. M.; Bai, Y. B.; Xian, M.; Shen, D. F.; Wang, Y. Q.; Du, S. R.; Lu, R.; Li, T. J.; Wu, Y.; Xu, W. Q. Supramol. Sci. 1998, 5, 599–602. (7) Serra, A.; Genga, A.; Manno, D.; Micocci, G.; Siciliano, T.; Tepore, A.; Tafuro, R.; Valli, L. Langmuir 2003, 19, 3486–3492. (8) Khomutova, G. B.; Gubin, S. P. Mater. Sci. Eng., C 2002, 22, 141–146. (9) Pike, J. K.; Byrd, H.; Morrone, A. A.; Talham, D. R. J. Am. Chem. Soc. 1993, 115, 8497–8498. (10) Pike, J. K.; Byrd, H.; Morrone, A. A.; Talham, D. R. Chem. Mater. 1994, 6, 1757–1765.
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In a slightly different approach, nanoparticles have been grown inside organic matrix Langmuir-Blodgett (LB) films by chemical insertion methods.11 Surface-functionalized gold nanostructured films have been obtained from gold colloids by choosing proper amphiphiles as templates. Zhong and co-workers have assembled gold nanostructures in organized molecular films of a series of gemini amphiphiles;12 hybrid nanocomposite films were fabricated by adsorption of gold nanoparticles on polymeric LB films.13,14 Swami et al. have demonstrated the formation of various anisotropic gold nanostructures by in situ reduction of chloroaurate ions, stabilized by surfactants at the air-water interface.15 In another approach, Ag, Au, CdS, PbS, and CdSe nanoparticles, etc., have been organized at the air-water interface exploiting electrostatic interaction between the precursor ions in the subphase and the oppositively charged Langmuir monolayers, followed by external chemical/photochemical reduction16 of the precursor compound.17-20 In a slightly different approach, Langmuir monolayers of alkyl amines and vitamin E have acted as reducing agents to synthesize nanoparticles and few reports (11) Mayya, K. S.; Patil, V.; Sastry, M. Langmuir 1997, 13, 2575–2577. (12) Zhong, L.; Jiao, T.; Liu, M. Langmuir 2008, 24, 11677–11683. (13) Endo, H.; Kado, Y.; Mitsuishi, M.; Miyashita, T. Macromolecules 2006, 39, 5559–5563. (14) Tanaka, H.; Mitsuishi, M.; Miyashita, T. Langmuir 2003, 19, 3103–3105. (15) Swami, A.; Kasture, M.; Pasricha, R.; Sastry, M. J. Mater. Chem. 2004, 14, 709–714. (16) Ravaine, S.; Fanucci, G. E.; Seip, C. T.; Adair, J. H.; Talham, D. R. Langmuir 1998, 14, 708–713. (17) Yi, K. C.; Horvolgyi, Z.; Fendler, J. H. J. Phys. Chem. 1994, 98, 3872–3881. (18) Pan, Z. Y.; Liu, X. J.; Zhang, S. Y.; Shen, G. J.; Zhang, L. G.; Lu, Z. H.; Liu, J. Z. J. Phys. Chem. B 1997, 101, 9703–9709. (19) Li, L. S.; Qu, L.; Lu, L. W. R.; Peng, X.; Zhao, Y.; Li, T. J. Langmuir 1997, 13, 6183–6187. (20) Urquhart, R. S.; Furlong, D. N.; Gengenbach, T.; Geddes, N. J.; Grieser, F. Langmuir 1995, 11, 1127–1133.
Published on Web 08/16/2010
DOI: 10.1021/la102371v
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explain modified reductant subphases.21,22 Due to its unique twodimensional reaction system in which the interfacial formation of nanostructures is carried out, the method opens new possibilities for morphological control mechanisms of organic-inorganic nanomaterials in novel interfacial reaction systems.23,24 Therefore, a single step in situ surface pressure induced controlled synthesis and fabrication of hybrid nanostructures in which the precursor amphiphile acts as an efficient templating agent warrants attention. Asymmetric decoration of nanoparticles with a hydrophobic chain at one end and a hydrophilic one at the other has enabled programmable self-assembly into hierarchical superstructures. While microfluidics, electrochemical and photochemical reduction, templating of porous membranes and nanotubes, and surfactant-aided growth have been employed for the anisotropic Janus nanoparticles, the complicated nucleation and growth made it difficult to control their structure and size.25-27 Polymer functionalized Janus particles were synthesized via “solid-state grafting-to” and “grafting-from” methods.28,29 Fujimoto et al. synthesized interfacial “unsymmetrical microspheres” taking advantage of regioselective surface modification.30 Janus gold nanoparticles were prepared by trapping nanoparticles within a Langmuir monolayer.31 Multistep interfacial ligand exchange processes32,33 were further employed; Pero et al. reported synthesis of Janus silica particles via protection and deprotection with polystyrene (PS) beads.34 Most recently, at the air-water interface, Janus microspheres formed a highly flexible and robust superhydrophobic membrane.35 Herein, we report a one-step templated 2D synthesis of Langmuir monolayers of amphiphilic Janus gold nanoclusters with controllable size and geometry using surface pressure as a thermodynamic variable. The two-dimensional nucleation and growth kinetics revealed an instantaneous hemispherical nucleation with edge growth followed by a surface pressure induced hierarchical 2D self-assembly of the interfacial Janus cluster monolayers, leading to sphere-rod-chain transition, and along with the exhibited insulator-to-metal transition.
Experimental Section Materials and Methods. Chloroauric acid, HAuCl4 3 3H2O (98.0% Aldrich), and tetraoctylammonium bromide, TOAB (98.0%, Aldrich), were used without further purification for the synthesis of amphiphilic electrostatic complexes. The Langmuir subphase was modified by L-tryptophan (99.0%, Merck) using ultrapure water (Millipore Academic, Resistivity, 18.2 MΩ.cm). (21) Swami, A.; Kumar, A.; Selvakannan, P. R.; Mandal, S.; Pasricha, R.; Sastry, M. Chem. Mater. 2003, 15, 17–19. (22) Zhang, L.; Shen, Y.; Xie, A.; Li, S.; Jin, B.; Zhang, Q. J. Phys. Chem. B 2006, 110, 6615–6620. (23) Khomutov, G. B. Adv. Colloid Interface Sci. 2004, 111, 79–116. (24) Moriguchi, I.; Tanaka, I.; Teraoka, Y; Kagawa, S. J. Chem. Soc., Chem. Commun. 1991, 19, 1401–1402. (25) Nie, Z.; Li, W.; Seo, M.; Xu, S.; Kumacheva, E. J. Am. Chem. Soc. 2006, 128, 9408–9412. (26) Roh, K. H.; Martin, D. C.; Lahann, J. Nat. Mater. 2005, 4, 759–763. (27) Walther, A.; M€uller, A. H. E. Soft Matter 2008, 4, 663–668. (28) Wang, B.; Li, B.; Zhao, B.; Li, C. Y. J. Am. Chem. Soc. 2008, 130, 11594– 11595. (29) Cheng, L.; Hou, G.; Miao, J.; Chen, D.; Jiang, M.; Zhu, L. Macromolecules 2008, 41, 8159–8166. (30) Fujimoto, K.; Nakahama, K.; Shidara, M.; Kawaguchi, H. Langmuir 1999, 15, 4630–4635. (31) Pradhan, S.; Xu, L. P.; Chen, S. Adv. Funct. Mater. 2007, 17, 2385–2392. (32) Xu, L. P.; Pradhan, S.; Chen, S. Langmuir 2007, 23, 8544–8548. (33) Glaser, N.; Adams, D. J.; B€oker, A.; Krausch, G. Langmuir 2006, 22, 5227– 5229. (34) Perro, A.; Reculusa, S.; Pereira, F.; Delville, M. H.; Mingotaud, C.; Duguet, E.; Lami, E. B.; Ravaine, S. Chem. Commun. 2005, 5542–5543. (35) Kim, S. H.; Lee, S. Y.; Yang, S. M. Angew. Chem., Int. Ed. 2010, 49, 2535– 2538.
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Chloroform (Uvasol, Merck) was used as the solvent for spreading the amphiphile on the air-water interface.
Fourier Transform Infrared Reflection-Absorption Spectroscopy (FT-IRRAS). FT-IRRAS in this work was done using a Bruker Optics Inc. (Germany) instrument customized for Langmuir monolayers, where a Vertex-70 FTIR spectrometer was coupled to the air-water reflection unit, A-511. For the LB films transferred on silanized quartz, the IR beam was directed with varied angles of incidence with an angular accuracy of (1. The mirror alignment was aided with a NIR source along with a glass beam splitter. The beam was polarized with a KRS-5 polarizer with a >98% degree of polarization. The spectrometer, the optical arms, and the MCT (mercury cadmium telluride) detector housing were purged with dry N2 gas before spectral acquisition with a resolution of ∼0.1 cm-1. Electrochemical Impedance. Measurements for the transferred LB multilayers were performed using the CHI-660B electrochemical workstation (CH Instruments, USA). Measurements were performed at E0 = 240 mV with a Pt wire counter electrode and Ag/AgCl reference electrode with 1 mM K4[Fe(CN)6] internal standard in 0.1 M KCl electrolyte solution. Scanning Electrochemical Microscopy (SECM). Measurements with a CHI 910 B bipotentiostat (CH Instruments, USA) followed acquisition of probe approach curves in the feedback mode. With a 25 μm diameter Pt tip electrode, the effect of the transferred monolayer on tip current was monitored. SECM current imaging was performed in the constant height amperometric feedback mode by scanning the tip over the substrate in a horizontal X-Y plane, keeping the tip-substrate distance “Z” constant and simultaneously monitoring the tip current as a function of position. Microscopic Techniques. Noncontact mode with an XE-100 AFM (Park Systems) instrument was used for the transferred LB films of the Au nanoclusters on to silanized Si(100) substrates. Scanning electron microscopic images were acquired with a FEI Quanta 200 instrument, and transmission electron microscopy was done with a JEOL 3010 electron microscope for the transferred monolayers on carbon coated copper films.
Interfacial Synthesis of Amphiphilic Janus Gold Nanoclusters. The TOAþ-AuCl4- complex was prepared in a 1:1
molar ratio in chloroform where the Au3þ ions were extracted from the aqueous layer by electrostatic complexation with TOAB. The in situ interfacial synthesis of amphiphilic gold nanoclusters was carried out using Langmuir-Blodgett technique by spreading 50 μL of 1 mM [TOAþ(tetraoctylammonium ion)-AuCl4-] precursor amphiphile in chloroform on an aqueous subphase (pH = 6.6) modified with L-tryptophan (0.01 mg/mL), where the reduction was restricted to the air-liquid interface at 25 C. The pressure-area (π-A) isotherms were acquired from a computer controlled double barrier Langmuir trough (KSV 5000 Finland). A Teflon trough of total area 772.5 cm2 was used with a dipping well at the center for transfer of films, while hydrophilic Delrin barriers were used for the compression-expansion of monolayers. Ultrapure water modified with amino acid L-tryptophan was used as the subphase for all monolayer studies with a Millipore-Academic system. The temperature of the subphase was controlled with a Julabo F-36 temperature controller with an accuracy of (0.1 C. Prior to each experiment, the trough was cleaned with chloroform, followed by methanol (extrapure, AR grade, SRL Fine chemicals, India) several times, and finally rinsed with ultrapure water. A total of 50 μL of chloroform solution of TOAþ-AuCl4- complex (1 10-3 and 5 10-3 M) was spread on the tryptophan (0.01 mg/mL) modified subphase using a Hamilton microsyringe. The Wilhelmy method with a platinum sensor of accuracy 0.1 mN/m was used to measure surface pressure at the air-water interface. The solvent was allowed to evaporate for 20 min before acquiring each isotherm. The monolayers were compressed at an optimized speed of 10 mm/min. Langmuir 2010, 26(17), 14047–14057
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Figure 1. (a) Schematic diagram showing the ideal Janus structure of the gold nanocluster formed in situ at the air-water interface and
(b) π-A isotherms for (i) 1 10-3 M TOAB at the water subphase, (ii) 1 10-3 M TOAþ-AuCl4- at the water subphase, and (iii) TOAþ-AuCl4- at the 0.01 mg/mL tryptophan subphase.
Surface Pressure Relaxation Experiments. For surface pressure relaxation experiments, the monolayers were compressed to a particular surface pressure, specified in graphs. At the constant surface pressure, the change in mean molecular area was monitored as a function of time. The relaxation experiments were performed for different concentrations of the gold precursor complex as well as different subphase tryptophan concentrations. Each plot for different surface pressures corresponded to separate experiments. All experiments were carried out by keeping isothermal conditions at room temperature (25 C).
Results and Discussion Interfacial Synthesis of Janus Gold Nanoclusters. We have demonstrated a novel method where orientation-specific amphiphilic Janus gold clusters were synthesized in an interfacially confined micellar media. The choice of appropriate surfacecapping ligands is critical in the control of the eventual particle structures and thereby in controlling the cluster capacitance properties. Suitable ligands were chosen on the basis of their hydrophobic-hydrophilic nature for imparting amphiphilic Janus character to the interfacial gold nanoclusters. The hydrophilicity of L-tryptophan as a protecting ligand as well as its capability to reduce Au3þ ions to Au0 have been exploited in the present investigation for the in situ synthesis of Janus gold nanoclusters at a biocompatible L-tryptophan modified water subphase. The ligands TOAB and L-tryptophan were geometry optimized with density functional calculations at the B3LYP/ 6-31 g (d, p) level of theory (see Supporting Information, Figure S1). The Janus character of the clusters was accrued from the two different hemispherical dielectric ligand shells having a common “N” binding to the cluster; the highly hydrophobic TOAþ protected the outer hemisphere, while the zwitterionic tryptophan in the subphase protected the other hemisphere, as depicted in Figure 1a. The much higher subphase tryptophan concentration compared to [AuCl4]- confined at the interface enabled tryptophan to further act as the protecting monolayer ligand, introducing hydrophilicity at the inner hemisphere of the cluster. 3þ L-Tryptophan reduced Au to Au0 inside the 2D hemimicelles formed from the reverse micellar aggregates of TOAþ-AuCl4complex, comprising the interior of the spherical micelle, vide Scheme 1. Quaternary ammonium surfactants are known to undergo surface micellization at interfaces.36 The critical packing parameter as 0.33 for the TOAþ-AuCl4- ion pair, calculated (36) Patrick, H. N.; Warr, G. G.; Manne, S.; Aksay, I. A. Langmuir 1999, 15, 1685–1692. (37) Acharya, S.; Efrima, S. J. Am. Chem. Soc. 2005, 127, 3486–3490. (38) Mark, P.; Nilsson, L. J. Phys. Chem. B 2002, 106, 9440–9445. (39) Vollhardt, D.; Retter, U. J. Phys. Chem. 1991, 95, 3723–3727.
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from the geometry optimized triangular pyramidal TOAþ/square planar AuCl4- (Scheme 1a), determined the preferred curvature of the aggregates with formation of surface micelles at the interface. As the spherical micellar solution of the gold precursor complex was sprayed at the tryptophan subphase, the spherical micelles changed into a hemimicellar form due to strong electrostatic and hydrophilic-hydrophilic interactions between [AuCl4]and the zwitterionic tryptophan in the aqueous subphase, driven by interfacial surface pressure. The π-A isotherms of the Janus nanocluster monolayers in Figure 1b show enhanced interfacial tension and molecular area. The large hysterisis in the π-A isotherm for the compression-expansion cycle in Figure 1b implied the irreversible nature of the nanocluster synthesis. The interfacially confined reduction provided a decreased activation energy path at room temperature, compared to the high temperature tryptophan reduction in bulk solution13 in view of the increased number density of the precursor amphiphiles inside the confined hemimicelles, in contact with the tryptophan subphase. With an increase in surface pressure, the decreased interparticle distance induced a higher surface density of the gold nanoclusters formed at the interface. The representative transmission electron micrographs of the hybrid gold nanoclusters for the monolayers transferred at various phase states are shown in Chart 1, indicative of the phase states of the TOAþ matrix assisted hybrid clusters. A gradual transition from a quasi gas (G) to liquid expanded (LE) to a liquid condensed (LC) phase and further to a solid state phase transition at the L-tryptophan modified subphase was evident from the π-A isotherm characteristics of the clusters. The cluster formation was independent of the concentration of the reductant tryptophan in the subphase, but depended strongly on the surface concentration of the spreading precursor amphiphile (see Supporting Information, Figure S2). Figure S2a illustrates the π-A isotherms shifting to larger areas with an increase in the subphase concentration. These results clearly indicate that the rate of interfacial reduction does not vary significantly with the surface amphiphile concentration. However, the number density of the clusters formed at the interface increased with spreading concentration. The isotherm for the 5 10-3 M concentration of the complex shows a clear variation in its compression features withstanding a much higher surface pressure (∼52 mN/m) but almost similar lift-off areas for the concentrations used at the tryptophan subphase, vide Figure S2b. A drastically varied phase structure reveals the critical roles played by the 2D surface pressure and the concentration of the surfactant amphiphile toward the interfacial self-assembly and higher order 2D structure organization. Hence, the size and the DOI: 10.1021/la102371v
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Biji et al. Scheme 1. a
a (a) B3LYP-6-31G* geometry optimized structure of TOAþ-AuCl4- complex showing its triangular basal plane at the air-water interface. The square planar [AuCl4]- complexed with TOAþ is shown above the plane of the figure. (b) Illustration of the mechanism of amphiphilic Janus gold nanocluster growth at the tryptophan modified subphase upon compression of the interfacial hemimicelles formed from the TOAþ-AuCl4- complex with a packing parameter P = V/la (hydrophobic volume/length of the hydrophobic chain hydrophilic head group area) = 0.33. (c) Hemimicellar growth of Janus gold cluster arrays at the air-water interface upon monolayer compression.
geometry of the in situ formed nanoclusters are mainly controlled by surface pressure for each amphiphile concentration. Apart from this, the effect of temperature on the rate of interfacial reduction was investigated upon collection of π-A isotherms of the in situ formed Janus gold nanoclusters at the tryptophan modified subphase (0.01 mg/L) at 10, 20, 30, and 40 C, and results are presented in Figure S2c. As the subphase temperature increased, the area occupied by the nanocluster Langmuir monolayer decreased, as did the surface pressure at the lift-off area. At reduced temperatures of the subphase, the 2D phase characteristics of the nanocluster film showed the presence of an additional phase. The appearance of the low temperature and surface pressure phase could be attributed to a reduced rate of formation of nanoclusters requiring larger activation energy for Au3þ f Au0 formation. Hence, during compression, the unreduced TOAþAuCl4- species phase behavior also became evident at lower temperature. Thus, with an increase in surface pressure, tuning the cluster size and geometry could be effectively achieved, which in turn could be used for studying various cluster size and geometry-controlled properties and applications. Mechanism of Interfacial Janus Cluster Formation. Based on the present investigations, Scheme 1 depicts the mechanism of interfacial Janus cluster formation supported by in situ isobaric relaxation kinetics, described in the following sections.40,41 The TOAþ matrix assisted Janus cluster monolayers remained metastable over the whole surface pressure region during interfacial reduction. In essence, cluster nucleation began even during the monolayer 2D gas phase state. Cationic, quaternary ammonium surfactants, such as, TOAB and TOAþ-AuCl4- spontaneously form inverse micelles in organic solvent and are known to undergo surface micellization at interfaces.36 From the π-A (40) Vollhardt, D.; Fainerman, V. B. J. Phys. Chem. B 2002, 106, 345–351. (41) Deng, J.; Viers, B. D.; EskerJay, A. R.; Anseth, W.; Fuller, G. G. Langmuir 2005, 21, 2375–2385.
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isotherm characteristics in Figure 1b, it was evident that a surface pressure induced hemimicellization occurred for TOAþ-AuCl4complex at the air-water interface. Consequently, as the spherical micellar solution of the gold precursor complex was sprayed at the tryptophan modified subphase, the spherical structure could rupture into hemimicellar forms due to strong hydrophilichydrophilic interaction between the [AuCl4]- counterions and the zwitterionic tryptophan and water molecules in the sub phase, driven by the interfacial surface pressure. As depicted in Scheme 1, the interfacial reduction of Au3þ to Au0 by tryptophan occurred inside the 2D hemimicelles followed by controlled aggregation toward the formation of hybrid amphiphilic Janus clusters. This was further confirmed by isobaric area relaxation kinetic studies which revealed the cluster growth to proceed through an instantaneous 2D hemispherical edge growth. Figure 2a depicts the well-defined surface plasmon bands of the transferred LB multilayer gold nanoclusters at 565 nm on a quartz substrate. The surface plasmon absorbance peak in the multilayer film appeared as a broad absorption in the range 500-675 nm and exhibited slight red shift of the λmax as the number of layers increased. These spectral changes could be attributed to the interparticle interactions of the close-packed gold nanoclusters and to the change in the refractive index of the film resulting in interparticle plasmon coupling that gave rise to a red-shift of the plasmon band with broadening. UV-vis absorption spectra of the gold nanorods formed for the 5 10-3 M concentration of TOAþ-AuCl4- at 15 mN/m surface pressure exhibited transverse (540 nm) and longitudinal (626 nm) surface plasmon absorbances for the multilayers (see Supporting Information, Figure S3). The intensity of the surface plasmon absorptions increased with an increase in the number of layers transferred. The existence of tryptophan on the cluster surface was revealed from the characteristic fluorescence emission of the transferred multilayers, vide Figure 2b. L-Tryptophan in aqueous medium showed a native fluorescence emission at ∼360 nm as shown in the inset of Langmuir 2010, 26(17), 14047–14057
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Chart 1. Hierarchical 2D Interfacial Self-Assembly of the Bifunctional Janus Gold Clusters Driven by Precursor Concentration and Surface Pressure, Showing Irreversible Aggregation of Unit Clusters
Figure 2b. The fluorescence emission spectra of the LB multilayer films of the transferred Janus nanoclusters at surface pressure 17 mN/m in Figure 2b clearly reveal the characteristic Janus structure, where the 15 L and 25 L as the odd number of layers on silanized hydrophilic quartz substrate showed substantial fluorescence intensity compared to the even number of layers. Since the multilayer transfer was done onto hydrophobic silanized quartz substrates (vide Figure 4a), the hydrophilic tryptophan protected cluster hemisphere was the peripheral surface for the odd layers with large fluorescence intensity. For even layers, the nonfluoreLangmuir 2010, 26(17), 14047–14057
scent TOAþ constituted the peripheral surface with negligible intensity. Validation of Janus Structure from Polarization-Modulated FT-IRRAS. The Janus structure was validated using angle dependent polarized FT-IRRAS, electrochemical impedance spectroscopy, and transmission electron microscopy. In FTIRRAS, the Janus structure analysis was done by detecting the orientation dependent vibrational changes in the adsorbed ligand functionalities. The spectral interpretation was achieved by fixing an optical model for the interaction of incident radiation with the DOI: 10.1021/la102371v
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Figure 2. (a) UV-vis optical absorption spectra and (b) fluorescence emission spectra excited at 290 nm for layer-by-layer transferred LB multilayers of the gold nanostructures on silanized quartz substrate. Inset shows the fluorescence emission spectra for pure L-tryptophan aqueous solution.
Figure 3. (a) Optical model for FT-IRRAS analysis of the LB Janus cluster monolayer transferred onto silanized ITO electrode. Parallel (p) (EII,x-z) and perpendicular (s) (E^,y) electric field vectors are shown for incident radiation in air phase. (b) Scheme showing the orientation of zwitterionic L-tryptophan on the Janus cluster surface (Au(111), hcp) derived from the FT-IRRAS spectral analyses. The red and black arrows indicate the transition dipole moment vectors of L-tryptophan, localized on the indole ring. Angle dependent polarized FT-IRRAS of the Janus LB film indicating (c) the presence of TOAþ (C-Nþ, νs) at the inner hemisphere of the Janus cluster for the grazing incidence (80) p-polarized IR beam on the cluster surface. (d) Presence and orientation of L-tryptophan at the peripheral hemisphere of the Janus cluster.
gold cluster as depicted in Figure 3a. The calculated conformational space of a single tryptophan molecule in water showed the major gauche (g-) conformation with the indole ring in an orientation perpendicular to the CR-Cβ bond, and 70% of all conformations were in a zwitterionic form.38 The polarization modulated angle dependent spectra of the cluster LB film on silanized quartz are depicted in Figure 3. In L-tryptophan, the transition dipole moment vectors lie in the indole plane, maximizing interaction with p-polarized radiation as illustrated in the Figure 3b. At an angle of incidence of 25 from the surface 14052 DOI: 10.1021/la102371v
normal, the p-polarized radiation was most sensitive, exhibiting NH3þ and the COO- functionalities, vide Figure 3c. The bands at 1470 and 1648 cm-1 were ascribed to the symmetric and asymmetric deformation modes of NH3þ, and 1723 and 1410 cm-1 to CdO and COO- stretching bands, respectively. However, at the grazing incidence angle 80, it failed to detect the fine tryptophan features owing to its specific orientation at the Janus cluster surface. In Figure 3d, the broad band at 1078 cm-1 is characteristic of TOAþ, the C-Nþ stretching frequency. Interestingly, the C-Nþ band appeared only for the p-polarized radiation at the Langmuir 2010, 26(17), 14047–14057
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Figure 4. (a) Surface anchoring of multilayer Janus clusters on silanized ITO electrode, (b) the corresponding equivalent circuits, (c) cyclic voltammograms and (d) Nyquist plots for the pretreated ITO, silanized ITO and the LB multilayers of amphiphilic Janus clusters transferred at 17 mN/m. ESI measurements were performed at E0 = 240 mV with Pt wire counter electrode, Ag/AgCl reference electrode with 1 mM K4[Fe(CN)6] internal standard in 0.1 M KCl electrolyte solution. Table 1. Impedance Spectral Data and Estimated Electron Transfer Rates for Janus Cluster Multilayers from the Modified Equivalent Circuit no. of layers
CDL (μF)
RCT (Ω)
RF (Ω)
CF (μF)
Zd (10-4 Ω)
kET (s-1)
ITO blank silanized ITO 4L 5L 6L 7L 8L
13.12 4.85 6.96 8.72 6.26 7.09 6.07
671.7 1773.0 810.8 140.3 886.9 303.9 1482.0
265.8 176.30 74.52 180.90 108.10 165.0
7.05 6.91 7.11 7.45 6.38 7.87
6.83 6.25 8.14 8.36 8.46 9.84 8.16
56.74 40.00 89.20 501.20 75.63 257.80 42.80
grazing incidence only, confirming the presence of TOAþ at the inner hemisphere of the transferred Janus cluster. For s-polarization, both near-normal and grazing incidence were unable to detect the TOAþ characteristic absorption. Janus Structure Induced Odd-Even Parity Effect in Nanocluster Multilayers. The Janus nature of the nanoclusters was further revealed from the odd-even parity effect exhibited by LB films of the multilayer clusters in charge transport studies with electrochemical impedance spectroscopy. Figure 4a shows the odd-even multilayer structure of the Janus clusters on the silanized ITO electrode whose electrochemical behavior in Figure 4c evidences the parity effect with reduced peak currents for the even layers. The Nyquist plots of the pretreated and silanized ITO and the odd-even layers of the amphiphilic Janus gold nanoclusters transferred at 17 mN/m are shown in Figure 4d using [Fe(CN)6]-3/-4 as a redox probe. For all multilayers, Nyquist plots feature semicircles as a combination of double layer capacitance, CDL, the apparent charge transfer resistance, RCT, kinetically controlled at intermediate frequencies, and the Warburg impedance, Zd, arising from diffusion of the redox couple toward the electrode interface. The equivalent circuit in Figure 4b is a parallel circuit between the ITO blank and the film contributions. The multilayer film is treated here as a resistor RF and a capacitor CF in parallel with a solution Langmuir 2010, 26(17), 14047–14057
resistance RS. A wide frequency range, 0.1 Hz to 100 kHz, was used with the internal Fe3þ/Fe2þ couple. For precise modeling, the Nyquist plots were analyzed with a modified equivalent circuit considering contributions from the film CF and RF to the total faradic impedance as shown in Figure 4b and Table 1 with much larger electron transfer rates for odd layers. Hierarchical 2D Self-Assembly in the Interfacial Janus Cluster Monolayers. Typical two-dimensional surface pressures prevalent within floating Langmuir films are on the order of a few dozen megapascals.37 Therefore, the interfacial reactions, including reduction, nucleation, and growth of the clusters, are strongly affected by surface pressure, which induces orientational order within the monolayer. Oriented attachment has been realized as a new way of crystal growth to transform the preformed nanoparticles into hierarchical assemblies. A remarkable feature of this approach was the direct formation of nanorod thin films from nanoparticulate thin films on a ligand/template. A sphere to 1D wire transition of the Janus gold nanoclusters was found as a result of interfacial compression of the clusters from the gas phase at 5 mN/m to its condensed phase at 23 mN/m at 25 C. A gradual increase in the cluster radius from 1.3 to 4.4 nm was observed during monolayer compression as a function of surface pressure, as depicted in Chart 1A; accordingly, the interparticle distance decreased from 4.9 to 0.8 nm, thereby increasing the metallic DOI: 10.1021/la102371v
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Figure 5. (a) Isobaric area relaxation curves obtained for 1 10-3 M TOAþ-AuCl4- complex on 0.01 mg/mL tryptophan subphase for
surface pressures 5, 10, 15, and 24 mN/m at 25 C and (b) corresponding surface pressure relaxation kinetic plots for different x values for the cluster monolayer at 10 mN/m. The insets show best fit for x with Vollhardt’s nucleation-growth model.
nature of the cluster monolayer. The as-formed clusters assembled in the form of 1D chains at larger surface pressures, 23 mN/m. The oriented attachment leading to formation of 1D wires was visualized in view of a finite dipole moment associated with an individual cluster and a directed Brownian motion governed by 2D surface pressure at the interface (see Supporting Information, Figure S5). In the present investigation, the phase changes associated with the bifunctional spherical clusters were interfacial, governed by the 2D surface pressure. In addition, due to the presence of a permanent dipole moment associated with the nanoclusters, their oriented attachment into a linear chain structure was viable. A directed Brownian motion of the spherically charged nanoclusters, governed by the 2D surface pressure at the interface, was therefore the driving force for the observed phase transition. In the context of nanorod formation in solution, it was reported that the presence of a permanent dipole moment in the nanoclusters could direct their oriented attachment into a linear chain. For the interfacially synthesized bifunctional nanoclusters of the present investigation, the absence of central symmetry induced a finite dipole moment to the clusters. This important parameter in conjunction with surface pressure and the interfacially modified Brownian motion of the clusters have been thought to impart specific ordering to the monolayer clusters forming nanorods, avoiding their fractal growth at the air-water interface. To address the kinetics of formation of nanorods starting from the primary bifunctional spherical nanoclusters, a general mechanism based on experimental facts was formulated. The two processes mainly conceived were (a) in view of their permanent dipole moment, with the oriented attachment of the interfacial nanoclusters resulting in intermediate, short-lived 1D wire aggregates and (b) surface pressure driven fast coalescence of such aggregates leading to nanorod formation. The transition of the nanorods into 1D wires suggests an electrostatic and surface pressure induced oriented attachment of individual nanospheres, followed by a faster coalescence of clusters in a representative linear chain at the interface. The nanosize of the particles contributed a larger extent of surface energy to the total energy of the system, which facilitated coalescence of the oriented and attached nanoclusters to nanorod formation. For the 5 10-3 M TOAþ-AuCl4complex at a low surface pressure of 15 mN/m, the spherical clusters phase transformed into rods with an aspect ratio of 4.6. At this surface pressure, they displayed a single crystalline Au(111) lattice with a hexagonal close packed arrangement with a lattice spacing of 2.71 A˚ (vide Chart 1B). The observa14054 DOI: 10.1021/la102371v
tions implied the growth direction of the rods to be in the (111) direction. At 30 mN/m, the rods maintaining almost the same aspect ratio as ∼4.9 organized with a certain order, forming end-to-end longitudinal chainlike structures, as shown in Chart 1B. For the charged bifunctional colloidal gold cluster systems with TOAþ and tryptophan protecting ligands, the interaction energy will consist of (i) the van der Waals attraction component along with (ii) the electrostatic contribution. Since the headgroup of the surfactant TOAþ is a quaternary amine, the cylindrical gold nanorod would have a large number of positive charges on its surface. Further, in view of the bifunctional Janus nature of the nanocluster, the tryptophan in zwitterionic form also remains attached to the gold nanorod, exerting electrostatic repulsion. Owing to the repulsive coulombic forces residing on the surface of the nanorod, a face-to-face orientation upon coalescence is hindered. However, the surface pressure being the major driving force for self-assembly exerts an end-to-end configuration (see Supporting Information, Figure S6). The aggregate curvature for a rod being more at the end regions ends up in a much lesser number of charged ligand functionalities, thereby enhancing the van der Waals attractive component between the two adjacent ends of the metal nanorods through the tetraoctyl chains. Overall, the dipole moment, the surface pressure, and the van der Waals attractive forces between the nanoclusters overtook the destructive Brownian motion, making stable nanorods followed by oriented end-to-end cluster attachment upon lateral compression of the Langmuir monolayers. Similarly, the condensed phase monolayer clusters at 20 mN/m in Chart 1C seem to have been aligned in a tubular form, because of an enhanced resultant dipole moment with much larger cluster density. On the basis of the present experiments, it can be inferred that pressure-induced oriented attachment has taken place in a similar manner to the alignment of nanowires, leading to a general route toward hierarchical organization of nanoscale building blocks into functional assemblies. Monolayer Relaxation Kinetics in the in Situ Formed Interfacial Janus Gold Nanoclusters. In order to explore the possible nucleation and growth mechanism for the interfacially formed Janus gold nanostructures at varied precursor and subphase tryptophan concentrations, surface pressure relaxation measurements were undertaken. Under normal spreading conditions, the relaxation processes lead to nucleation and growth of 3D structures which disturb the homogeneity and integrity of the spread molecular monolayers and cause in-plane discontinuities and rearrangement transformations into a 3D phase, thereby leaving preformed nuclei. Overall nucleation and growth of the in situ monolayers has been kinetically modeled by classical theories Langmuir 2010, 26(17), 14047–14057
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Figure 6. (a) 3D depiction of the effect of precursor concentration on the transformation constant as a function of surface pressure and (b) TEM and AFM images illustrating unit geometries for the hemispherical edge growth as well as the 1D rodlike growth of the hybrid gold nanoclusters transferred at 15 mN/m for 1 10-3 and 5 10-3 M TOAþ-AuCl4- concentrations, respectively.
where the generalized formalism applied to monolayers was represented by Vollhardt and Retter as39 A0 - A ¼ 1 - exp - Kx ðt - ti Þx A0 - A¥
ð1Þ
where A is the molecular area at any time t, A0 is the initial molecular area, and A¥ is the molecular area in the limit of infinite time. The overall transformation constant Kx and induction time ti are constants. The value of time exponent x depends on the geometrical conditions concerning the shape and means of growth of the 3D nuclei as well as on the kinetics of growth. Langmuir 2010, 26(17), 14047–14057
At the beginning of the surface pressure relaxation experiment, the nucleation begins and the nuclei grow freely in the monolayer, while their growth rate is limited mainly by the diffusion rate of the monolayer material into the 3D nanostructures. The linearized form of eq 1 as (ln[1/{1 - (A0 - A)/(A0 - A¥)}])1/x = -Kx1/x(t - ti) provided a convenient form of presenting area relaxation data, controlled by nucleation mechanisms upon plotting (ln[1/{1 - (A0 - A)/A0 - A¥)}])1/x versus t. Thus, different nucleation mechanisms could be distinguished by the time exponent x from linear regression. Following this formalism, Figure 5 represents the representative relaxation curves and relaxation kinetic plots for selected surface pressures corresponding to 1 10-3 M DOI: 10.1021/la102371v
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Figure 7. (a) SECM probe approach curves for the transferred Janus cluster monolayers at different surface pressures onto insulating quartz substrates, exhibiting insulator-metal transition. Working electrode: SECM tip, Pt (25 μm); counter electrode, Pt wire; and reference electrode, Ag/AgCl with 1 mM K4[Fe(CN)6] in 0.1 M KCl electrolyte solution. (b) Energy-well model showing insulator-metal transition for monodisperse nanocluster arrays as a function of surface pressure. d (=2r) is the diameter of the cluster, δ is the intercluster distance, and E is the energy barrier with E1 > E2 > E3. (c) SECM current images of the insulating quartz and a 30 layer Janus cluster on quartz.
TOAþ-AuCl4- complex. Though the nucleation-growth theory of Volhardt et al. is mainly used for unfolding the nucleation around collapsed monolayer phases of the amphiphilic molecules, the present investigation makes use of this kinetics in view of the interfacial nucleation of nanoclusters beginning from the gas phase. This is further accounted by the fact that the TOAþ matrix assisted hybrid Janus cluster monolayers are metastable over the whole surface pressure region associated with continuous interfacial reduction. For in situ interfacial reduction, the amphiphilic gold precursors gradually underwent reduction and thereby the gold clusters formed started to nucleate even during the initial stages of compression of the monolayer, even before the isobaric value was obtained. Relaxation measurements were carried out at different surface pressures, keeping the surface pressure constant for an individual curve and measuring the evolution of the molecular area with time for different concentrations of 1 10-3, 5 10-3, and 10 10-3 M TOAþ-AuCl4- (see Supporting Information, Figure S4). Individual and independent experiments were conducted for each isobaric area relaxation measurement for each different concentration of the gold precursor, as well as the subphase concentrations. The percentage of reduction in total area gradually increased with the surface pressure. However, unlike standard molecular amphiliphilic monolayers reported, the relaxation of the hybrid nanocluster monolayers commenced even at lower surface pressures, indicating the metastable nature over the entire surface pressure region. At larger concentrations of both precursor spreading TOAþ-AuCl4- as well as the subphase reductant tryptophan, the best fit was obtained with the value of time exponent, x = 3/2, corresponding to an instantaneous nucleation mechanism with edge growth. Consequently, very high nucleation rates and an increase in the overall transformation rates at 14056 DOI: 10.1021/la102371v
increasing surface pressures were determined by the growth rates. At higher surface pressures, nucleation and growth rates of the 3D nuclei were similar but more rapid; however, the situation changed dramatically as the concentration of the TOAþ-AuCl4- complex increased from a hemispherical growth to a 1D rod transition. Compared to 1 10-3 and 10 10-3 M concentrations of TOAþ-AuCl4-, the 5 10-3 M concentration monolayer showed higher nucleation rate on a 0.01 mg/mL tryptophan subphase. In Table S1 (see the Supporting Information), the isobaric area relaxation kinetic parameters form various concentrations of the TOAþ-AuCl4- precursor complex on varied subphase tryptophan concentrations are summarized. For the 5 10-3 M concentration of TOAþ-AuCl4-, the time exponent’s best fit value as x = 1 indicates a 1D rodlike cluster geometry. For both 1 10-3 and 5 10-3 M precursor concentrations at larger surface pressures, further growth of the 3D phase did not lead to a homogeneous multilayer coverage due to the formation of a surface pressure induced network of large scale, long chainlike structures. Hence, at larger surface pressures, while for a 1 10-3 M precursor the spherical clusters rearranged into a quasi-1D wire, the 5 10-3 M precursor self-organized into end-to-end attached single crystalline 1D rods. Figure 6 shows the overall effect of precursor concentration on the nucleation transformation constant at different surface pressures. As evident from the figure, the sphere-to-rod transition gradually increased with an increase in the precursor concentration. However, at very high concentrations, the transition was found to be hindered due to the presence of larger surface density of the spherical clusters formed by hemispherical nucleation. The latter was found to dominate over the expected sphere to rod transformation with 10 10-3 M TOAþAuCl4-. As the overall transformation constant Kx involves the nucleation and growth rates, its increase with surface pressure Langmuir 2010, 26(17), 14047–14057
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implies an enhancement in the nucleation and growth rates of the aggregate assemblies at the interfacial monolayer. Figure 6b corresponds to representative TEM and AFM images of the gold nanoclusters transferred onto Si(111) substrates at 15 mN/m for 1 10-3 and 5 10-3 M precursor concentrations, respectively. Insulator-Metal Transition in Janus Gold Nanocluster Langmuir Monolayers. Electron transport in the oriented Janus cluster arrays was studied by scanning electrochemical microscopy (SECM). Consequently, steady-state measurements were made with a 25 μm Pt microelectrode tip. Characteristic phase dependent electrostatic intercluster coupling, as shown in Figure 7a, was noted in accordance with an increase in the core radii and a decreased interparticle distance at larger surface pressures, vide Chart 1A. Figure 7b represents an energy well model for nanocluster arrays as a function of surface pressure, illustrating G, LE f LC f S phase transitions. As the interparticle distance, δ, decreases, the energy barrier, E, for each embedded cluster decreases, enhancing the intercluster coupling. For δ > 10 A˚, the electronic states are localized on individual clusters in the film, although the electric fields of the clusters couple via classical dipolar coupling.42,43 With δ < 10 A˚ as in the condensed (23 mN/m) and liquid condensed (17 mN/m)) monolayer states, the wave functions of adjacent particles overlap and undergo exchange coupling, decreasing the coulomb gap, a particle sizedependent barrier to charge delocalization. Further reduction in δ resulted in disappearance of the gap, initiating an insulator-tometal transition in the clusters, attaining a metal-like behavior as depicted in the SECM probe approach curves in Figure 7a. SECM current imaging of the transferred multilayers is shown in Figure 7c indicating the metallic nature of the film. Such a phenomenon was observed with in situ polyaniline monolayers44 and in alkane thiol protected silver45 and gold nanocrystals.46
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Langmuir strategy. The cluster growth occurred inside the surface pressure driven TOAþ-AuCl4- hemimicelles, where the subphase tryptophan acted as the reductant as well as the capping agent at the lower hemisphere of the clusters at the air-water interface. The Janus structure was validated using angle dependent polarized FT-IRRAS, where orientation dependent vibrational changes in the adsorbed ligand functionalities were detected. A clear orientational odd-even parity effect with the odd number of layers showing a much higher electron transfer rates confirmed the Janus structure of the interfacial clusters, evidenced from the electrochemical impedance measurements of the transferred Janus layers. Isobaric area relaxation investigations further evidenced toward a hemispherical instantaneous nucleation with edge growth mechanism of the nanoclusters formed at the tryptophan subphase. The hemimicellar transition preceding the equilibrium Janus structure formation depended on the amphiphile geometry, the precursor concentration, and the hydrophobic/hydrophilic ligand ratio. Thus, control over interfacial activity was demonstrated by tuning the particles’ amphiphilicity. The Janus cluster synthesis in the confined micellar media significantly increased the interfacial tension, and the oriented interfacial cluster array was found both electronically and electrochemically active. Surface pressure as a thermodynamic variable effectively controlled the interparticle separation; intercluster electron coupling exhibited insulator-metal transition in the Janus cluster monolayers. In view of the in-built anisotropy in the Janus cluster monolayers, controlled functional architectures are envisaged with novel electronic attributes. The present investigation illustrated that varying experimental conditions could change the dimensionality and orientation of the assemblies and directionality in self-assembles could contribute to the design aspects of nanoelectronics.
Conclusions Oriented, amphiphilic, and fluorescent Janus gold clusters, establishing the Janus character in terms of ligand asymmetry and distribution, were synthesized interfacially through a one-pot in situ hemimicellar assisted method, based on the efficient
Acknowledgment. The work was supported by the Department of Science and Technology, Government of India, Grant No. SR/S2/CMP-57/2006.
(42) Markovich, G.; Collier, C. P.; Henrichs, S. E.; Remacle, F.; Levine, R. D.; Heath, J. R. Acc. Chem. Res. 1999, 32, 415–423. (43) Liljeroth, P.; Vanmaekelbergh, D.; Ruiz, V.; Kontturi, K.; Jiang, H.; Kauppinen, E.; Quinn, B. M. J. Am. Chem. Soc. 2004, 126, 7126–7132. (44) Zhang, J.; Barker, A. L.; Mandler, D.; Unwin, P. R. J. Am. Chem. Soc. 2003, 125, 9312–9313. (45) Quinn, B. M.; Prieto, I.; Haram, S. K.; Bard, A. J. J. Phys. Chem. B 2001, 105, 7474–7476. (46) Liljeroth, P.; Quinn, B. M.; Ruiz, V.; Kontturi, K. Chem. Commun. 2003, 13, 1570–1571.
Supporting Information Available: Geometry optimized structures of the ligands, π-A isotherms as a function of subphase and precursor amphiphile concentrations and effect of temperature, UV-vis spectra of transferred multilayers of the in situ formed nanorods, mechanisms of sphere-rod transitions, isobaric area relaxation curves and relaxation kinetics plots. This material is available free of charge via the Internet at http://pubs.acs.org.
Langmuir 2010, 26(17), 14047–14057
DOI: 10.1021/la102371v
14057
